The present invention relates to a power switching device which has fixed and movable contacts and is used to control the supply of electric power by causing the movable contact to make and break contact with the fixed contact. More particularly, the invention relates to such a power switching device having a stable breaking performance.
Referring to FIGS. 1-4, there is shown a conventional power switching device. As shown in FIG. 1, a mount 1 made of steel plate is provided with a plurality of holes 1a (FIG. 3) for installing the mount on the body of the switching device. A base 2 of insulating material is securely fixed to the mount 1 with screws 3 (FIG. 3). Installed on a fixed iron core 4, which is provided by laminated silicon steel plates, is an operation coil 5. A leaf spring 6 serving as a shock absorber is mounted in the gap formed between the core 4 and the mount 1. A movable iron core 7, which is disposed in opposition to the fixed core 4, is attracted to the core 4 when the coil 5 is energized. An insulating crossbar 8 is connected to the core 7 via a pin 9. A trip spring 10 (FIG. 2) disposed between the crossbar 8 and the mount 1 normally urges the crossbar upward to open the main circuit of the switching device. A movable contactor 11 incorporating a movable contact 11a which is inserted into a hole 8a (FIG. 2) of the crossbar 8 and is secured by a spring 12. A fixed contactor 13, which includes a fixed contact 13a opposed to the contactor 11 and to the contact 11a, is securely fixed to a terminal 15 with screws 14. The terminal 15 is secured to the base 2 with screws 16 and 17.
An arc runner 13b is electrically connected to the fixed contactor 13, but it is also possible to make the runner integral with the contactor 13. A terminal screw connected with the electric wire of the main circuit is joined to the terminal 15. An arc box 19 made from insulating material and securely fixed to the base 2 with screws 20 (FIG. 2) includes a top portion 19b and a side plate 19c. The box is provided with holes 19a, through which gas which can cause arcing is expelled. A deionizing grid 21 made from magnetic material is formed in a shape as shown in FIG. 4. A commutator electrode 22 is securely fixed to the top portion 19b of the arc box 19. The movable contact 11a and the fixed contact 13a are disposed within the arc extinguishing chamber of the device.
In the operation of the power switching device described above, when a voltage is applied to the operation coil 5 in the open-circuit condition of the main circuit, magnetic flux is developed between the fixed core 4 and the movable core 7, whereby the core 7 is attracted to the core 4 against the resilience of the spring 10. At the same time, the crossbar 8 operates in the same manner, and the movable contact 11a comes into contact with the fixed contact 13a thereby closing the main circuit. Thereafter, when the coil 5 is deenergized, the contact 11a is moved away from the contact 13a, resulting in arcing in the region as indicated by character A in FIG. 1 between the both contacts.
Referring next to FIGS. 5A-5F, the movement of the arc beginning with its generation and ending with the break of the current will now be described. (Since the arc extinction chamber is symmetrical, FIGS. 5A-5F show only one half of the chamber.) FIG. 5A shows that the contact 11a in contact with the contact 13a. Then, when the contact 11a is moved away from the contact 13a, an arc 23 is generated, as shown in FIG. 5B. The distance between the contacts becomes large with time until a certain distance is reached. The current flowing through the contactors 11 and 13 and the deionizing grid 21 drive and expand the arc 23, as shown in FIG. 5C. As a result, one end of the arc 23 is transferred from the surface of the contact 13a to the arc runner 13b, as shown in FIG. 5D. Then, dielectric breakdown takes place between the protrusion of the arc 23 resembling a circular arc (FIG. 5D) and portion B of the runner 13b (FIG. 5E) and the one end of the arc 23 is transferred to the portion B of the runner. Then, the other end of the arc 23 is transferred from the fixed contact 11a to the commutator electrode 22, as shown in FIG. 5F and, at the same time, the arc is attracted into the grid 21 and extinguished, thus completing the breaking operation.
The prior art power switching device thus constructed has some drawbacks. First, as one end of the arc 23 tends to stay on one end portion of the contact 13a and the other end on the surface of the contact 11a, the contacts 13a and 11a, which must be made from costly materials, are worn rapidly. Secondly, the arcing time is prolonged, making the energy lost in the arc quite high. Thus, it is impossible to break a large current.
In order to eliminate these disadvantages, an improved power switching device has been proposed. This improved device is similar to the conventional device of FIGS. 1-.varies.except for its commutator electrode and arc runner. Referring to FIG. 6, such an improved commutator electrode 22 is shown. This electrode is provided with a space M and a planar portion N opposed to the deionizing grid 21. As can be seen from FIG. 6, the contact 11a and contactor 11 enter the space M in the electrode 22. That is, the device is so constructed that when the distance between the contacts reaches a maximum value, the commutator electrode 22 is located between the electrodes.
Referring next to FIGS. 7A and 7B, the arc runner 13b is brazed to a U-shaped fixed contactor 13, and the wall thickness Y1 of the runner is so selected that it is greater than the wall thickness X1 of the fixed contact 13a.
The movement of the arc 23 in this improved device will be described with reference to FIGS. 8A-8F. FIG. 8A shows the contact 11a in contact with the contact 13a. When the contact 11a is disengaged from the contact 13a, an arc 23 is developed between the contacts, as shown in FIG. 8B. As the distance between the contacts becomes large, both ends of the arc are rapidly transferred from the contacts to the commutator electrode 22 and to the runner 13b, respectively, as shown in FIG. 8C. The arc 23 is driven by the current flowing through both contactors 11 and 13. When transferred, the arc is acted upon by a strong magnetic field as indicated by arrow B in FIG. 6, the field being caused by both the current flowing through the contactor 11 and the electrode 22 made of magnetic material. At this time, a driving force F is generated which moves the arc 23 from the contact 11a to the electrode 22. The force F and the shape of the commutator electrode hasten this transfer. Since the runner 13b is made from magnetic material, the arc 23 is attracted to the runner 13b, and the leg of the arc is rapidly transferred from the contact 13a to the runner. Then, the arc 23 is driven and expanded by the current flowing through the electrode 22 and the contactor 13, as shown in FIG. 8D, and thereafter it assumes the condition as shown in FIG. 8E. Finally, it is extinguished in the deionizing grid, as shown in FIG. 8F, completing the breaking operation.
In this manner, in the improved device, one end of the arc is transferred very rapidly from the movable contact 11a to the commutator electrode 22, and thus the contact 11a is worn away slowly. Further, the arcing time is shortened, thus reducing the arc energy. Hence the break performance is improved.
However, this improved device also has a disadvantage. In particular, the runner 13b is brazed to the contactor 13. The coupling ratio between the runner 13b and the contactor 13 expressed in terms of a percentage of the truly coupled area to the apparently coupled area is typically as low as 60% or so. In other words, the runner is actually only partially coupled to the contactor 13 due to voids in the braze. Further, it is impossible to control the positions at which the elements are not brazed together, and therefore every brazing provides different locations at which the runner 13b and contactor 13 are not brazed together. In uncoupled portions 24, as indicated by the hatching in FIG. 9, current will flow through the contactor 13 and then flow through the runner 13b in the direction indicated by the arrow in FIG. 9. Thereafter it flows into the arc 23, driving the arc in the direction away from the grid 21. Thus, the improved device is sometimes unable to effectively break the arc current.